1. Field of the Invention
This invention relates to a photodiode for a light receiving module of optical communications, in particular, a photodiode with wavelength selectivity suitable for wavelength division multiplex (WDM) optical communications. The WDM communications signifies an optical communications system making use of more than one wavelength of light as signal light. For example, a first wavelength xcex1 is allocated to transmission (upstream) light and a second wavelength xcex2 is assigned to receiving (downstream) light. In this case, the photodiode (PD) of a receiving module on a subscriber should preferably not feel xcex1 but sense only xcex2. Conventional photodiodes, however, have sensitivity both for xcex1 and xcex2.
This application claims the priority of Japanese Patent Application No.2001-84542 filed on Mar. 23, 2001 which is incorporated herein by reference.
2. Description of Related Art
FIG. 1 shows a cross-sectional view of a typical one of conventional photodiodes (PD). This is a bottom incidence type photodiode. The material of a light receiving layer depends upon the wavelengths of signal light. For example, in the case of the optical communication systems using a wavelength band ranging from 1.3 xcexcm to 1.6 xcexcm, an InGaAs layer 2 is epitaxially grown directly or indirectly on an n-InP substrate 1 as a light receiving layer. InGaAs is simplified expression of a ternary mixture crystal In1xe2x88x92xGaxAs. Here, x is a mixture rate. The mixture rate is determined to be a definite value from the lattice matching condition between the InP substrate and the InGaAs light receiving layer. A p-type region 3 and a pn-junction 4 are produced by diffusing Zn, a typical p-type dopant, into a central part of the n-type light receiving layer 2.
The pn-junction has ends revealing on the top surface. The revealing ends are covered with an insulating film 6 (passivation film), e.g., of silicon nitride (SiNx). A p-type electrode 5 is formed upon the p-type region 3. An annular n-type electrode 7 with a central opening is made upon the bottom of the n-InP substrate 1. An antireflection film 8 is laminated upon the opening of the InP substrate 1. A photodiode is reversely biased by applying a lower voltage to the p-type electrode 5 (anode) and a higher voltage to the n-type electrode 7 (cathode) in use. The reverse bias induces depletion layers on both sides of the pn-junction 4, a p-type depletion layer on the p-side and an n-type depletion layer on the n-side. The n-type depletion layer on the n-side is important. Signal light 9 goes via the antireflection film 8 into the bottom of the InP substrate 1, and attains to the n-type depletion layer in the light receiving layer. Light having energy larger than a band gap makes pairs of electrons and holes by inducing a band gap transition of electrons from a valence band up to a conduction band. An electric field formed by the reverse bias pulls holes upward over the pn-junction into the p-type region 3 and pushes electrons downward to the n-type region, which induces photocurrent. The photocurrent is taken out from the photodiode. The band gap of the light receiving layer determines what wavelengths of light can be detected by the photodiode.
FIG. 2 shows wavelength dependence of sensitivity of the photodiode having an InGaAs light receiving layer. The abscissa is a wavelength (xcexcm). The ordinate is sensitivity (A/W). The InGaAs photodiode has a wide range (Q) of sensitivity from 1 xcexcm(P) to 1.6 xcexcm(R). The InGaAs photodiodes are endowed with high utility and prevalence by the wideness of the sensitivity range.
The wide sensitivity range incurs a problem on the photodiodes in the case of the multiwavelength optical communications which includes a plurality of wavelengths of light signals. Conventional InGaAs photodiodes have sensitivity for not only the object wavelength xcex2 but also the noise wavelength xcex1 to which the PD should not response.
For example, in the case of a single fiber bidirectional, optical communications system making use of a 1.3 xcexcm wavelength (xcex1) and 1.55 xcexcm wavelength (xcex2), 1.3 xcexcm light emitted from a laser diode (LD) is noise to a 1.55 xcexcm-detecting PD. The 1.3 xcexcm light emitted from the laser diode tends to go into the photodiode, which induces noise in the 1.55 xcexcm-detecting photodiode. The above is a problem appearing in a bidirectional transmission system in the full duplex transmission mode
Otherwise, a rapidly developing dense wavelength division multiplexing (DWDM) system or coarse wavelength division multiplexing (CWDM) system contains a plurality of channels and uses a set of different wavelengths with a narrow spacing for one direction stream of signals for the channels and another set of different wavelengths with a narrow spacing for the other direction stream of signals for the channels. The DWDM and the CWDM require sophisticated laser diodes which can produce many different wavelenghs of light which are rich in monochromacity. The photodiodes having a wide range of sensitivity as shown in FIG. 2, are not favorable for the DWDM and the CWDM, because the PD would invite serious crosstalk among neighboring channels. On the contrary, the photodiodes having a narrow range of sensitivity with fine resolution (xcex94xcex) of a nanometer to tens of nanometers are suitable for suppressing crosstalk among different channels with small differences of wavelength. An important problem is how to give photodiodes sharp wavelength selectivity for meeting the requirement of suppressing the crosstalk among the channels.
One way of assigning wavelength selectivity is an addition of a dielectric multilayered film to a wide sensitivity range photodiode. A wavelength selective photodiode is obtained by adding a dielectric multilayered film on the market to the opening of the bottom of the wide sensitivity range PD of FIG. 1.
{circle around (1)} Masahiro Mitsuda, Tohru Nishikawa, Tomoaki Uno, Masato Ishino, xe2x80x9cOptical Cross-talk Reduction of LD/PD Module for ATM-PON Systemxe2x80x9d, Proceedings of the 2000 Communications Society Conference of IEICE, B-10-55, p278, proposed an LD/PD module employing a double-cladding optical fiber, adding a 1.3 xcexcm absorbing InGaAsP layer on the top of a photodiode, laying the photodiode epi-down on a glass substrate, painting an enclosure of the photodiode with a stray light absorbing resin for suppressing crosstalk. The double cladding prevents once fiber entering LD light (xcex1) from leaking out of the fiber. The InGaAsP absorption layer fitted to the top of the photodiode absorbs 1.3 xcexcm light which is noise for the photodiode. The painted resin eliminates the 1.3 xcexcm noise light from the photodiode.
{circle around (1)} employs three different means for eliminating noise light from the photodiode.
{circle around (2)} R. Momura, H. Yanagisawa, A. Goto, Y. Fukutomi, N. Kitamura, M. Kunitsugu, K. Kaede,xe2x80x9d An ONU Transceiver Module using PLC for 622 Mb/s downstream ATM-PON systemxe2x80x9d, Proceedings of the 2000 Communications Society Conference of IEICE, B-10-59, p282, proposed a transceiver (LD/PD) module which protects a photodiode from invasion of LD light by a WDM (wavelength division multiplexing) filter. The WDM filter is a dielectric multilayered film made on a glass substrate having wavelength selectivity.
The dielectric multilayered film is made by piling repeatedly in turn at least two kinds of dielectric layers with different refractive indexes and different thicknesses on the glass substrate. The on-glass dielectric film has wavelength selective reflection and wavelength selective transparency. Reflection wavelength and transparent wavelength are determined by the choice of refractive indexes and thicknesses of dielectric layers. Arbitrary wavelengths can be assigned to the reflection wavelength and the transparent wavelength by selecting refractive indexes and thicknesses of the component dielectric layers. The reflection rate and the transparency are enhanced by increasing the number of repetitions of piling of a pair of dielectric layers. Dielectric multilayered filters on the market are all glass-based filters produced on independent glass substrates. On-glass dielectric multilayered films on sale have drawbacks of a size as large as and a price as high as a photodiode module itself at present. It takes much time and higher cost to install the external, independent on-glass dielectric multilayered film in front of the photodiode. Coupling of the photodiode module to the on-glass dielectric multilayered film would raise cost and volume in a great measure.
Another prior art which should be described here is an absorption layer built-in a photodiode which invented before by the same inventors as the present invention. The absorption layer has no direct relation to a novelty of the present invention. Since some embodiments will make use of the known absorption layer, references of the absorption layer are here cited.
{circle around (3)} Japanese Patent Application No.9-256107 (256107/""97), xe2x80x9cPhotodiode and Photodiode Modulexe2x80x9d, (Japanese Patent Laying-Open No.11-83619) proposed a photodiode having an InGaAsP absorption layer made on either a top surface or a bottom surface of a substrate for absorbing noise xcex1 light.
{circle around (4)} Japanese Patent Application No.11-260016(260016/""99), xe2x80x9cPhotodiodexe2x80x9d, (Japanese Patent No.3046970) proposed a photodiode having two InGaAsP absorption layer made on both top and bottom surfaces of a substrate for absorbing noise xcex1 light.
The above two references suggested photodiodes including built-in absorption layers for noise xcex1 light. The inventors who are the same inventors as the present invention were aware that a semiconductor with a band gap wavelength xcexg absorbs the light of shorter wavelength xcex than xcexg (xcex less than xcexg) but does not absorb the light of longer wavelength xcex than xcexg (xcex greater than xcexg). The inventors hit upon an addition of a layer of the bandgap wavelength xcexg which satisfies inequality xcex1  less than xcexg less than xcex2 to a photodiode for annihilating noise xcex1 light. The absorption layer carried in the photodiode is suitable for a single fiber bidirectional optical communications. In the case of an ONU (subscriber site) of communication systems using xcex1=1.3 xcexcm (upstream) and xcex2=1.55 xcexcm(downstream), an absorption layer of xcexg=1.35 xcexcm to 1.5 xcexcm, in particular xcexg=1.4 xcexcm is advantageous. A quaternary mixture crystal InxGa1xe2x88x92xAsyP1xe2x88x92y corresponds to a band gap wavelength of xcexg=1.35 xcexcm to 1.5 xcexcm.
Prevalence of optical communications overall in the world requires size reduction and cost reduction of PD, LD or LD/PD modules. A photodiode itself should be miniaturized further.
One purpose of the present invention is to provide a noise-resistant photodiode which has an inherent function of suppressing cross talk by repulsing noise light. Another purpose of the present invention is to provide a photodiode having a built-in wavelength selective filter on an opening through which signal light goes in. Another purpose of the present invention is to provide a low-cost, small-sized, noise-resistant photodiode.
The present invention proposes a photodiode having a built-in dielectric multilayered film formed on a resin film deposited on a surface through which signal light goes in for repulsing noise xcex1 light. The built-in dielectric multilayered film and the intermediate resin characterize the present invention. Complicated dielectric multilayered films, in general, cause strong inner stress. Strong inner stress prevents a photodiode from wearing the complicated dielectric multilayered film directly. The intermediate resin has enough elasticity for reducing inner stress induced by the built-in dielectric multilayered film. The problem of the inner stress is described.
The conventional photodiode (PD) of FIG. 1 has the antireflection film 8 deposited directly on the aperture of the bottom of the substrate 1 for alleviating reflection of signal light. The antireflection film can be produced by evaporation or sputtering. The antireflection film 8 is a kind of dielectric multilayered films. The purpose of the antireflection film is not to reflect the light (signal light; xcex2) to be sensed so much. A simple condition of suppressing the reflection of the signal light is imposed upon the design of the antireflection film. Since the condition is simple, a small number of the dielectric layers is enough to form the antireflection film. Piling of different materials induces inner stress in the film and the substrate. Inner stress increases in proportion to the number of the dielectric layers. The small number of dielectric layers protects the antireflection film from inner stress.
The antireflection film is made by piling a few transparent dielectric thin layers of MgF2, SiO2, Al2O3, TiO2, Zr2O3, SiON, Ta2O5 or Nb2O5 less than ten layers in turn on an InP substrate or an InGaAs layer by sputtering or evaporation. Even a single layer can play the role of the antireflection film. Two to six layers usually construct the antireflection film. The simple condition of reducing reflection of only a wavelength saves the number of dielectric layers constructing the antireflection film. Weak inner stress allows depositing the antireflection film directly on the substrate.
A wavelength selective filter which separates two wavelengths with a narrow difference xcex94xcex should satisfy far more difficult conditions than the antireflection film. The smaller the wavelength difference xcex94xcex decreases, the more the number of the necessary layers increases. A dielectric multilayered film for wavelength selection requires a pile of tens to hundreds of dielectric layers. Such a set of many different dielectric thin layer is usually made on an independent rigid glass substrate which is high resistance against inner stress. Independent wavelength selective dielectric multilayered filters made on glass substrates are on the market as a passive optical part. It is unfavorable to deposit such a thick pile of tens to hundreds of dielectric layers on a PD substrate directly. Differences of thermal expansion among the PD semiconductor and the different dielectric layers induce distortion, exfoliation or other degradation of the dielectric multilayered filter through an increase of inner stress. Thus, direct deposition to the PD chip of tens to hundreds of dielectric thin layers is impossible.
The present invention solves the problem of distortion, exfoliation or other degradation by depositing an elastic transparent resin film on a surface of a photodiode and forming a dielectric multilayered film on the elastic transparent resin. The elastic transparent resin intermediates between the photodiode semiconductor and the dielectric layers. The elasticity of the resin film absorbs differences of the thermal expansion coefficients and alleviates inner stress occurring between the photodiode chip and the dielectric layers. The elastic resin protects the upper dielectric multilayered film from transforming or peeling off from the PD semiconductor. The dielectric multilayered film is nearly free from inner stress owing to the intervening resin film. The resin film can absorb non-uniformity of the semiconductor surface. The gist of the present invention is a three-storied, stress-alleviating structure of semiconductor/resin/dielectrics. Available variations of resins, dielectric films and types of photodiodes are preliminarily described for clarifying the scope of the present invention.
[Resins]
A resin film is produced on a semiconductor (substrate or film) by spin-coating a semiconductor (substrate or film) with a transparent resin fluid and hardening the resin into a thin resin film by heating or UV-irradiation. Suitable resins are polyimide, fluoric polyimide, benzocyclobutene(BCB), deuteride silicone resins, or siloxane polymer. The resins have sufficient elasticity after hardening. Thick, hard dielectric films are deposited on the resin film by evaporation or sputtering. The elasticity of the resin film absorbs differences of thermal expansion between the semiconductor and the thick dielectric layers.
[Dielectric Multilayers]
A wavelength selective dielectric film is built by repeatedly piling in turn at least two kinds of transparent dielectric thin layers of, for example, MgF2, SiO2, Al2O3, TiO2, Zr2O3, SiON, Ta2O5 or Nb2O5 which is transparent to signal light of xcex2. These materials have different refractive indices. A pile of thin layers of different refractive indexes and different thicknesses generates desired wavelength selective reflection. Dielectric multilayers piled upon a glass substrate have been well known as a wavelength selective filter. A semiconductor device carrying the rigid pile of the dielectric multilayers is novel. These oxides have all strong rigidity itself. A direct pile of the oxides on a semiconductor would cause big inner stress, large distortion, transformation, exfoliation or breakdown of the PD itself. The intermediate resin enables the present invention to build the dielectric multilayered film in the photodiode for the first time.
[Fabrication of Films]
The resin film is made by spin-coating an object surface of a semiconductor (bottom substrate or top light receiving layer) with a material resin fluid and thermohardening or UV-hardening the resin film. Spin-coating is a convenient method which adjusts a film thickness by varying the rotation speed. The way of hardening (thermohardening or UV-hardening) depends upon the inherent property of the object resin. Sufficient elasticity remains in the resin after hardening.
The dielectric multilayered film is produced by evaporating, sputtering or chemical-vapor-depositing (CVD) at least two oxides above-described of definite thicknesses repeatedly in turn on the resin film. The wavelength selective filter consists of the resin film and the dielectric multilayered film. Photolithography can reform a once fabricated wavelength selective filter into an arbitrary shape.
The hardening temperature of the thermohardening resins is lower than the temperature of heating steps included in the wafer process. Formation of the (thermohardening) resin film causes no degradation of the devices made in the preceding wafer process. The fact allows the present invention to make the wavelength selective filters at a stroke for all chip parts on the wafer on the whole at a final stage of the wafer process. This is an important advantage of the present invention. After the fabrication of the wavelength selective filters, the wafer is divided into individual photodiode chips by scribing crosswise and lengthwise along cleavage lines. The present invention is preferable for mass production, which cuts cost down.
Fortunately, the present invention scarcely changes the shape and the size of the photodiode, because no independent part is assigned to the photodiode. Mounting the photodiode chip on a silicon bench of the PD module is similar to a conventional one.
[Fabrication of Lens]
The intervening resin favors the present invention with another important advantage. Amorphous resin fluid allows a photodiode to make a converging lens on the bottom of a substrate. A dielectric multilayered film requires a definite incidence angle (right angle incidence or so) for displaying the predetermined wavelength selection performance. Discrepancy of the incidence angle incurs extra penetration of noise light. Thus, a base for the dielectric multilayered film should be a flat surface. A curved surface of the lens is unfavorable for direct deposition of the dielectric multilayered film. The intermediate resin film enables the present invention to make a built-in lens on a substrate by preparing a flat surface upon the curved surface. Although the substrate is fabricated into a curved (lensed) surface, fluidity of the resin material liquid buries the curvatures so long as the thickness of the resin is larger than the height of the built-in lens. Spin-coating allows a resin material fluid to cover the lensed surface into a flat plane. The flat surface of the resin film serves a preferable flat base for producing a dielectric multilayered film thereon.
The built-in lens has a function of converging incidence light effectively at a sensing region (depletion layers on the pn-junction). Indium phosphide (InP) has a high refractive index of n=3.5. Even a low height lens of InP has a short focal length and a strong converging function. The use of a built-in lens is less expensive than mounting of a individual, separated lens. The lens converges light to a small spot at the pn-junction, which enhances the efficiency of the O/E conversion. Further, the lens enables the photodiode to narrow the sensing region (pn-junction, p-type region). A conventional sensing region has a diameter of 100 xcexcm to 150 xcexcm. A sensing region can be reduce under 100 xcexcm, e.g., 80 xcexcm to 50 xcexcm in the lens-formed photodiode of the present invention. The most favorable case allows a sensing region of a diameter of 30 xcexcm. The narrow sensing region favors a photodiode with high speed response (e.g., more than 2.5 Gbps) by lowering the electrostatic capacitance. In the lensed photodiode, the resin film plays the role of absorbing the curvature forming a lens on the semiconductor substrate and preparing a flat base for the dielectric layers in addition to alleviation of inner stress due to discrepancy of thermal expansion between the dielectric layers and the semiconductor.
[Materials of Photodiodes]
This invention can be applied to a photodiode having an InP substrate, another photodiode having a Si substrate and a further photodiode having a GaAs substrate. An Si-PD and a GaAs-PD sense visible light. A PD on InP senses near infrared light. In the case of a photodiode based upon an InP substrate, the substrate is an n-type InP, a p-type InP or a semi-insulating (SIxe2x80x94) InP.
The photodiodes based on the InP substrates have a light receiving layer of a ternary mixture crystal of InxGa1xe2x88x92xAs or a quaternary mixture crystal of InxGa1xe2x88x92xAs1xe2x88x92yPy. An InGaAs photodiode has a definite sensitivity range from 1 xcexcm to 1.6 xcexcm as shown in FIG. 2. InGaAsP photodiodes can be endowed with various sensitivity ranges which are changed by controlling the mixture rates of x and y in InxGa1xe2x88x92xAs1xe2x88x92yPy.
Basic component layers of a photodiode are a substrate and a light receiving layer. Optionally, a window/capping layer can be added on the light receiving layer. The window/capping layer has functions of suppressing dark current, reducing recombination of electrons and holes at the surface of the light receiving layer and raises sensitivity. The window layer and capping layer are equivalent layers. But the names change in accordance with the structure of a photodiode. In the case of a top incidence type, it is called a window layer. In the case of a bottom incidence type, it is called a capping layer.
Furthermore, a buffer layer can be optically inserted between the substrate and the light receiving layer. A full-component example has an (n-, p-, or SIxe2x80x94) InP substrate, an InP buffer layer, an InGaAs or InGaAsP light receiving layer and an InP window/capping layer.
[Directions of Light]
A surface of a photodiode having the sensing region (pn-junction) is defined as a xe2x80x9ctopxe2x80x9d surface. The other surface of the photodiode is defined as a xe2x80x9cbottomxe2x80x9d surface. Photodiodes are classified by the direction of light into a xe2x80x9cbottom incidencexe2x80x9d, a xe2x80x9ctop incidencexe2x80x9d and a xe2x80x9cfront end incidencexe2x80x9d types.
A bottom incidence type means a photodiode into which light goes via the substrate bottom. In the bottom incidence type, one annular electrode is formed on the bottom and the other electrode is formed overall on the top sensing region above the pn-junction. The resin film and the dielectric film are produced on an epitaxial wafer in the wafer process before separating into chips in this case.
A top incidence type means a photodiode into which light goes via the top. In the top incidence type, one electrode is overall formed on the bottom and the other electrode is formed in an annulus on the top sensing region above the pn-junction. The resin film and the dielectric film are produced on an epitaxial wafer in the wafer process before separating into chips also in this case.
A front end incidence type means a photodiode into which light goes via the front end in parallel with the top and the bottom surfaces. In the front end incidence type, one electrode is overall formed on the bottom and the other electrode is also overall formed on the top sensing region above the pn-junction. The resin film and the dielectric film are produced on a front end of an isolated photodiode chip after separation.
This invention can be applied all to the top, bottom and front end incidence types of photodiodes. The bottom incidence type and the top incidence type are, in particular, preferable for the present invention.
[Addition of Epitaxial Absorption Layers]
The present invention aims at proposing a photodiode which is sensitive for signal light xcex2 to be sensed but insensitive to the other light (noise) xcex1 to be rejected.
Besides the wavelength selective filter, an addition of a semiconductor layer which absorbs noise xcex1 light is effective for annihilating crosstalk. The xcex1 absorption layer is not novel, since the same inventors as the present invention had invented as described before. xcexg denotes a band gap wavelength of a semiconductor. A semiconductor with a band gap wavelength xcexg which satisfies xcex1 less than xcexg less than xcex2 absorbs xcex1 but allows xcex2 to pass through. An InGaAsP (xcexg) layer can be a xcex1 absorption layer for a photodiode with an InGaAs light receiving layer.
In the case of photodiode with an InGaAsP light receiving layer which absorbs xcex2, another InGaAsP (xcexg) layer can be assigned to be a xcex1 absorption layer. Mixture ratios are different for the InGaAsP of the light receiving layer and the InGaAsP of the absorption layer. The light receiving layer InGaAsP has a band gap wavelength longer than xcex2. The absorption layer InGaAsP has a band gap wavelength shorter than xcex2 but longer than xcex1. Two parameters allow quaternary mixture crystal InGaAsP to make a xcex1 absorption layer and a light receiving layer satisfying lattice fitting condition.
Such an absorption layer can be assigned to either a top surface or a bottom surface of a substrate for absorbing noise light of xcex1. It is more effective to add two absorption layers on both surfaces of the substrate. The improvement excludes doubly the noise xcex1 light by the xcex1 absorption layer and the wavelength selective filter. The modes of exclusion are different for the two devices. The dielectric multilayer excludes noise light by reflecting. Noise reflection rate increases in proportion to the number of dielectric layers. Hundreds of layers are required for reflecting perfectly noise light. However, reflected noise light survives and returns to the photodiode again and again by being reflected by parts or walls of a package. Besides, the reflection rate has strong dependence upon the incidence angle. The dielectric multilayer cannot prevent slanting stray noise from invading into the photodiode. Exclusion by the dielectric multilayer is imperfect in some cases.
On the contrary, an epitaxially-grown xcex1 absorption layer absorbs noise light. Noise light does not survive. Slanting stray noise light is also absorbed by the xcex1 absorption layer. Absorption has no dependence upon the incidence angle.
When the strength of noise light is weak, the dielectric multilayered film is sufficient for excluding noise. When noise light is strong, an addition of the absorption layer is effective to eliminate noise completely.
Advantages of the present invention are described.
(1) The elastic intervening resin film protects the dielectric multilayered film from transforming and exfoliating by alleviating inner stress and distortion.
(2) Current photolithography enables the present invention to produce the wavelength selective filter easily.
(3) Plenty of wavelength selective filters can be made on a wide wafer at a stroke in the wafer process before separation into chips. Mass production lowers cost of fabricating the photodiodes.
(4) The size and the shape of photodiodes are scarcely changed by the addition of the resin film and the dielectric layers. The manner and condition for mounting the photodiode are similar to that of the conventional photodiodes.
(5) If a converging lens is made on the semiconductor surface, a resin film conveniently gives a flat plane as a base on which the dielectric films are deposited.